Substrates like glass are used in a variety of applications such as architectural applications, automotive applications, aircraft applications, etc. Oftentimes, the substrates must be coated with a functional coating(s) to exhibit the required performance properties. Examples of functional coatings include electroconductive coatings, photocatalytic coatings, thermal management coatings, hydrophilic coatings, etc.
A photocatalytic coating can be applied on, for example, a glass substrate, to help keep the surface of the glass free of common organic surface contaminants. The photocatalytic coating works in the following manner: when the photocatalytic coating is exposed to light, the coating absorbs photons, generates electron-hole pairs and, in the presence of water or moisture, generates highly reactive hydroxyl radicals that tend to oxidize organic materials on the coated substrate. Ultimately, any organic material on the surface of the coated substrate gets converted to a more volatile, lower molecular weight material that can evaporate away or be washed away.
Titania (TiO2) is well known in the art as a material that has good photocatalytic properties. Conventionally, a TiO2 coating is a polycrystalline layer made up of numerous crystallites having the same crystal structure. In a coating composition, TiO2 can be present in one of the following crystal structures: anatase, rutile and brooktite. Depending on the crystal structure of TiO2 present in the coating, the coating will exhibit different performance properties, i.e. photocatalytic activity, UV induced hydrophilicity, durability, etc. For example, a coating made up of TiO2 crystallites having the anatase crystal structure has a higher photocatalytic activity than a coating made up of TiO2 crystallites having the rutile or brooktite crystal structures.
Polycrystalline functional coatings like TiO2 coatings can be deposited on a substrate using a variety of techniques. For example, well known techniques such as spray pyrolysis, chemical vapor deposition (“CVD”) and magnetron sputtered vacuum deposition (“MSVD”) can be used to deposit a TiO2 coating on a glass substrate. The advantage of spray pyrolysis and CVD over other techniques for depositing a TiO2 coating on a glass substrate is they can be utilized on a float glass line during a glass production process. A further benefit of a CVD process is that it utilizes heat in the glass substrate as a result of the glass making process to degrade CVD precursors and form a durable coating. If the TiO2 coating is applied on-line during the glass making operation, the TiO2 coated glass can be produced at a reduced cost due to the efficiency of the process.
Regardless of the process used to deposit a polycrystalline functional coating on a glass substrate, there is no process available at the current time for depositing a functional coating made up of a certain material like TiO2 under fixed deposition conditions, wherein the deposited functional coating can comprise a first polycrystalline film made up predominantly of material having a first crystal structure and a second polycrystalline film made up predominantly of the same material having a second crystal structure. For example, there is no process currently available for depositing a photocatalytic coating comprising TiO2 on a float glass line under standard operating conditions, wherein the deposited functional coating comprises a first polycrystalline film made up predominantly of TiO2 having the rutile crystal structure and a second polycrystalline film made up predominantly TiO2 having the anatase crystal structure.
The present invention provides a method for depositing a functional coating comprising a material that can be present in more than one crystal structure over a substrate, wherein the functional coating is made up of one or more films comprised predominantly of a material having different crystal structures. The method of the invention can be used to deposit multi-film functional coatings which are designed for optimization of at least one property such as photocatalytic activity, durability, etc.